In general, a nitrile-based rubber is a polymer which is prepared by copolymerization of a conjugated diene-based monomer, such as butadiene, and an ethylenically unsaturated nitrile-based monomer such as acrylonitrile.
It is known that a nitrile-based rubber has excellent oil resistance due to the use of an ethylenically unsaturated nitrile-based monomer including a highly polar nitrile group, specifically, acrylonitrile (ACN), during the preparation. Also, the amount of ethylenically unsaturated nitrile included in the nitrile-based rubber affects various vulcanization properties of the nitrile-based rubber. Specifically, in a case in which ACN is used as the ethylenically unsaturated nitrile-based monomer, when the amount of the ACN in the nitrile-based rubber is increased, resistance to hydrocarbon fuels and petroleum-based fluids is increased, and heat resistance, ozone resistance, wear resistance, tensile strength, hardness, and density are increased, but low temperature flexibility, resilience, and gas permeability are decreased.
Nitrile-based rubbers are classified into 5 grades including low nitrile (amount of ACN in the rubber: 18 to 20%), medium and low nitrile (amount of ACN in the rubber: 28 to 29%), medium nitrile (amount of ACN in the rubber: 33 to 34%), high nitrile (amount of ACN in the rubber: 38 to 39%), and ultra-high nitrile (amount of ACN in the rubber: 45 to 48%), according to the amount of the ethylenically unsaturated nitrile-based monomer, e.g., ACN. In general, the high nitrile and ultra-high nitrile are used in areas in which resistance to hydrocarbon having a high aromatic content, such as gasoline, is required. Also, the medium nitrile is used in a case in which swelling of a rubber product is not an issue or oil has a low aromatic content. The low nitrile and the medium and low nitrile are mainly used in the preparation of liquids having an insignificant swelling effect, such as paraffin oil or polyalphaolefin, or foamed insulation materials and hoses requiring low temperature characteristics such as low temperature resistance.
Typically, during the polymerization of a nitrile-based rubber having an amount of ACN of 34 wt % or less, ACN having high reactivity is first consumed in the polymerization process, and a butadiene (BD) block (azeotropic composition of AN:BD=34:66, 10° C.) is generated when a conversion rate into the nitrile-based rubber is 60% or more. However, the generation of the butadiene block in the nitrile-based rubber causes the extension of the range of glass transition temperature (Tg) of the nitrile-based rubber, generates multiple Tgs, and furthermore, results in the reduction of resilience and low temperature characteristics.
In order to address such limitations, a method of suppressing the generation of the butadiene block by dividedly adding ACN during the polymerization of the nitrile-based rubber has been suggested. However, the processability and low temperature characteristics of the nitrile-based rubber may be improved by the above method, but tensile stress and stiffness of the rubber may be reduced.
Thus, there is a need to develop a method for preparing a nitrile-based rubber having improved mechanical properties as well as excellent processability and low temperature characteristics.